1. Field of the Invention
The present invention relates generally to rotary actuators, and more particularly to method and apparatus for balancing a rotary actuator by selectively modifying weight distribution characteristics of the rotary actuator for locating an installed head stack assembly portion center-of-gravity along a pivot axis.
2. Description of the Prior Art
A representative disk drive incorporating the structures referred to herein is taught, for example, in U.S. Pat. No. 5,953,183 to Butler et al., entitled xe2x80x9cHEAD STACK ASSEMBLY FOR A MAGNETIC DISK DRIVE WITH A PASS-THROUGH FLEX CIRCUIT CABLE,xe2x80x9d and is incorporated herein by reference.
A disk drive includes a disk having a track, a disk controller for generating a servo control signal, and a head stack assembly. The head stack assembly includes a rotary actuator and a flex cable assembly. The rotary actuator includes an actuator body portion, a pivot axis extending through the actuator body portion, and a head extending from the actuator body portion. The rotary actuator is responsive to the servo control signal for positioning the head over the track.
The flex cable assembly includes a flex cable, a flex clamp and electrical components (e.g., an integrated circuit containing a pre-amplifier). The flex cable is interposed between a printed circuit board assembly and the integrated circuit. The head stack assembly includes a rotatable head stack assembly (HSA) portion which includes a rotatable flex cable portion for receiving the servo control signal and the rotary actuator. As such, the rotatable HSA portion includes those subcomponents of the head stack assembly which are configured to rotate about the pivot axis.
It is known that gravitational effects on a mass of the attached flex cable portion and other components of the head stack assembly produce a torque on the rotary actuator about an axis which extends perpendicularly from the pivot axis. Such a torque is undesirable as the rotary actuator becomes susceptible to an external acceleration torque being applied to the rotary actuator about the pivot axis when the disk drive is exposed to an external linear acceleration. The external linear acceleration may be due to vibration or shock for example. The resulting external acceleration torque results in positional errors of the heads relative to the desired tracks (off-track errors) being introduced into the system during operation. Off-track errors are particularly undesirable for a number or reasons. In particular, such errors directly impact the overall seek time of the disk drive because the settling time will increase. Further, the data transfer rate will decease due to the off-track errors. Moreover, as the tracks per inch (TPI) specification of disk drives increase, the system sensitivity of such off-track errors likewise increases.
There have been attempts to mitigate against gravitational effects of the mass of the flex cable and other components which produce a torque applied to the rotary actuator. In this regard, a known methodology includes estimation of which portion of the flex cable is associated with a gravitational related torque applied to the rotary actuator related to the mass of the flex cable, in addition to the mass other components of the head stack assembly. For example, half of the flex cable may be a rough estimate. The prior art method would call for cutting the flex cable at the estimated location and removing the rotary actuator with the severed portion of the flex cable. The center of gravity of the rotary actuator with the severed portion of the flex cable would then be determined by mechanical means. Using such center of gravity information, the rotary actuator design would then be modified so as to attempt to locate the center of gravity of the rotary actuator with the severed portion of the flex cable at the pivot axis. Such prior art rotary actuator designs have proven to include a high degree of off-track errors associated with external linear acceleration acting on the disk drive. Accordingly, there is a need in the art for a method and apparatus for making an improved rotary actuator.
An aspect of the invention can be regarded as a method of balancing a rotary actuator for use in a rotatable head stack assembly (HSA) portion in a disk drive. The rotary actuator has a pivot axis, and the rotatable HSA portion has an installed HSA portion center-of-gravity. The method provides for determining a desired rotary actuator center-of-gravity location for locating the installed HSA portion center-of-gravity along the pivot axis for mitigating acceleration of the rotary actuator about the pivot axis due to external linear acceleration experienced by the disk drive during a track-follow operation. The method further provides for measuring weight distribution characteristics of the rotary actuator to determine an actual center-of-gravity of the rotary actuator. The method further provides for selectively modifying weight distribution characteristics of the rotary actuator to locate the actual center-of-gravity of the rotary actuator at the desired rotary actuator center-of-gravity location.
In an embodiment of the present invention, the step of selectively modifying weight distribution characteristics of the rotary actuator may include adding a balancing mass to the rotary actuator. Further, such adding of the balancing mass may include dispensing a material upon the rotary actuator to locate the actual center-of-gravity of the rotary actuator at the desired rotary actuator center-of-gravity location. The material has a first liquid phase when dispensed and has a second solid phase subsequent to being dispensed. Further, the step of selectively modifying weight distribution characteristics of the rotary actuator may include removing a mass from the rotary actuator. Such removing of the mass may include drilling the rotary actuator.
Another aspect of the invention can be regarded as a rotary actuator balancing system for use with a rotary actuator. The balancing system is provided with a support plate having upper and lower sides thereof. The upper side is sized and configured to receive the rotary actuator thereon in a weight supporting relationship therewith. The balancing system is further provided with a force measurement device in mechanical communication with the support plate. The force measurement device is sized and configured to sense weight distribution characteristics of the rotary actuator when supported by the support plate for determining an actual rotary actuator center-of-gravity of the rotary actuator. The balancing system is further provided with a mass modifying device sized and configured to modify weight distribution characteristics of the rotary actuator to locate the actual rotary actuator center-of-gravity at a desired rotary actuator center-of-gravity location. The balancing system is further provided with a mass modifying device support sized and configured to selectively move the mass modifying device with respect to the support plate. The mass modifying device support is disposed in mechanical communication with force measurement device.
In an embodiment of the present invention, the force measurement device includes three strain gauges. Further, the lower side of the support plate includes three datum points, and the three strain gauges are sized and configured to respectively contact the three datum points in mechanical communication therewith. The mass modifying device may be a material dispensing unit sized and configured to add mass to the rotary actuator. The material dispensing unit may be sized and configured to dispense a material. The material may have a first liquid phase when dispensed and have a second solid phase subsequent to being dispensed. Further, the mass modifying device may be a material removal unit sized and configured to remove mass from the rotary actuator. The material removal unit may be a drill. The rotary actuator may be provided with a bore extending therethrough. The support plate may include a bore datum post extending from the upper side thereof. The bore datum post is sized and configured to extend through the bore of the rotary actuator for locating the rotary actuator relative to the support plate. Further, the support plate may have an angular orientation feature extending from the upper side thereof. The angular orientation feature is sized and configured to contact the rotary actuator for locating the rotary actuator relative to the support plate. In addition, the rotary actuator may have a coil portion and a tang extending therefrom. The angular orientation feature may be a tang registration pin extending from the upper side of the support plate. The tang registration pin is sized and configured to contact the tang of the rotary actuator for locating the rotary actuator relative to the support plate. The support plate may include an elevation location feature extending therefrom. The elevation location feature is sized and configured to contact the rotary actuator in spaced relation from the upper side of the support plate for locating the rotary actuator relative to the support plate.